For a good while this process was just straight up not possible for C and C++, until someone put in a lot of effort to recreate the bootstrap process for reasons of reproducible builds (and it's still a long and complicated process I think only a few people have done). For decades people were just building from source with compilers that had extremely long and undocumented bootstrap chains (they may have documentation for how to bootstrap from a different C or C++ compiler, but there wasn't one that would start from scratch).